Coffee extract solutions are used to produce instant coffee powders or concentrated coffee extract solutions which are obtained by concentrating coffee extract solutions. However, it is known that coffee extract solutions are extremely unstable and, in particular, contact of a coffee extract solution with oxygen causes the components of coffee in the solution to be oxidized in a short period of time, resulting in the flavor and fragrance of the coffee being considerably degraded. In order to prevent degradation of the flavor and fragrance of coffee extract solutions, various methods have already been proposed, for example, a method in which a large amount of sugar, e.g., sucrose or glucose, is added to a coffee extract solution, and another method in which a small amount of ascorbic acid (vitamin C), which is an anti-oxidizing agent used as a food additive, is added to a coffee extract solution. However, the method in which sugar is added causes the coffee extract solution to become excessively sweet or undesirably makes the solution into a high-calorie product. In regard to the method that utilizes vitamin C, it is reported that the coexistence of a coffee extract and ascorbic acid causes Chinese hamster cells to show mutagenicity which is four or more times as high as that in the case of a coffee extract solution alone. Therefore, coffee products obtained by this method give people an unfavorable impression.
According to still another proposed method, liquefied carbon dioxide gas or dry ice is added in a container filled with a coffee extract solution to replace the head space in the container with a carbon dioxide atmosphere, thus stabilizing the coffee extract solution. However, it is known that, since oxygen has already been dissolved in the coffee extract solution, autoxidation of the coffee extract solution cannot be prevented simply by blowing carbon dioxide gas into the head space, and the solution is thus oxidized add degraded with time.
Although attempts have been made to add sugars, vitamin C and the like to coffee extract solutions or concentrated coffee extract solutions for the purpose of stabilizing the quality of such solutions as described above, no satisfactorily results have yet been obtained. Further, the proposed method in which a coffee extract solution or a concentrated coffee extract solution is filled in a hermetically sealed container and the head space in the container is replaced with an atmosphere of an inert gas such as carbon dioxide gas is still incapable of satisfactory preventing degradation of the quality of the stored solution.